1. Field of the Invention
The present invention relates to a liquid crystal composition and, more particularly, to a liquid crystal composition suitably used in a liquid crystal device controlled by two-frequency addressing scheme.
2. Description of the Prior Art
Liquid crystal devices have been used in a variety of applications for television sets, computer terminals, and office equipment. A display device in such equipment is constituted by a liquid crystal display device having a large number of pixels. These pixels are arranged in a matrix form and multiplexing driven. However, when the number of pixels is increased in such a liquid crystal device, the number of scanning lines and hence the number of time-divisional operations are increased. As a result, sufficiently high contrast of the ON and OFF pixels cannot be obtained by a cross effect phenomenon or the like.
A conventional liquid crystal device of this type can also be used in a printer having small optical shutters for controlling light transmission and shielding of liquid crystal elements. In this case, these optical shutters are arranged in a line or a few lines to control light transmission, and characters and images are formed by a large number of small light spots. The size of the optical shutter is very small in such a liquid crystal device (e.g., 0.1 mm.sup.2 or less). Characters and images of one page are constituted by a very large number of points. The optical shutters must be driven at very high speed in order to achieve a practical printing speed for printing a 10-page (A4 size) document per minute. However, the conventional driving method cannot sufficiently satisfy such requirements.
Two-frequency addressing scheme utilizing a dielectric dispersion phenomenon is known to solve the above problem. According to this driving technique, a high-frequency (e.g., 100 kHz) signal voltage is applied to a liquid crystal to orient the axes of liquid crystal molecules in a direction perpendicular to an electric field, and a low-frequency (e.g., 200 Hz) signal voltage is applied to the liquid crystal to orient the axes in a direction parallel thereto.
According to the two-frequency addressing scheme, both light-transmitting and light-shielding behaviors of liquid crystal molecules of the liquid crystal device are controlled by the different electric fields, and therefore, the liquid crystal device can be operated at high speed.
A typical example of a display device driven by the two-frequency addressing scheme is disclosed in U.S. Pat. No. 4,236,155. Liquid crystal material compositions used in such display devices are disclosed in Japanese Patent Disclosure (Kokai) No. 58-118886 and U.S. Pat. No. 4,550,981. According to the two-frequency addressing scheme, the liquid crystal device is operated at a relatively high speed, and the cross effect phenomenon can be restricted to improve contrast. However, the liquid crystal composition used in this display device does not have properties which satisfy a high-speed response in the printer.
The liquid crystal shutters in the printers driven by the two-frequency addressing scheme and liquid crystal compositions used in these liquid crystal shutters are disclosed in Japanese Patent Disclosure (Kokai) Nos. 57-83577 and 57-5780 and U.S. Pat. Nos. 4,559,161 and 4,609,256. When the liquid crystal shutters are driven by the two-frequency addressing scheme, they can be operated at a relatively high speed.
Liquid crystal compositions to be applied in liquid crystal devices driven by the two-frequency addressing scheme are disclosed in Japanese Patent Disclosure (Kokai) No. 57-5782, U.S. Pat. Nos. 4,566,759, 4,460,770, and 4,387,038, and GB Patent No. 2085910.
These liquid crystal compositions, however, contain two or three benzene rings and/or cyclohexane rings as their major constituents. These liquid crystal materials have small absolute values of dielectric anisotropy and high cross-over frequency fc for "0" dielectric anisotropy .DELTA..epsilon.. Therefore, the absolute value of dielectric anisotropy is small and/or the cross-over frequency is high
Response characteristics of the liquid crystal composition used in the liquid crystal device depend mainly on a value of dielectric anisotropy .DELTA..epsilon., a viscosity, and an elastic constant of the composition. More specifically, the larger the absolute value of dielectric anisotropy .DELTA..epsilon.becomes, the quicker the liquid crystal molecules respond. In addition, since the liquid crystal molecules can react in a weak electric field, a low drive voltage can be used. The lower the viscosity becomes, the shorter the response time becomes. The smaller the elastic constant becomes, the shorter the response time becomes.
An RF current can be made small at a low cross-over frequency. For this reason, power consumption can be reduced, and at the same time, dielectric heat generation due to capacitance and Joule heat generation due to resistance in the device can be prevented. In addition, the arrangement of the display driver can be made simple.
However, no existing single liquid crystal compounds satisfy all the conditions described above. A desired liquid crystal material is prepared by mixing different liquid crystal compounds, each having at least one of the desired properties. Various liquid crystal compositions prepared in this manner have the following disadvantages. Even if some compositions have low cross-over frequencies, they have small absolute values of dielectric anisotropy. Even if some compositions have large absolute values of dielectric anisotropy, they have high cross-over frequencies. Even if some liquid crystal compositions have large absolute values of dielectric anisotropy, low cross-over frequencies, and low apparent viscosities, they have large elastic ratios associated with high-speed response. As a result, they are not suitable for high-speed operation.
In order to solve the above problem, the present applicant proposed a liquid crystal composition obtained by mixing a compound having an ester bond and two or three benzene rings and/or cyclohexane rings with a four-ring compound having an ester bond, four benzene rings and/or cyclohexane rings, and a cyano group at the terminal, as described in U.S. Ser. No. 762,615. This liquid crystal composition exhibited a typical dielectric dispersion phenomenon. At the same time, the composition has a large absolute value of dielectric anisotropy, a low viscosity, and a low elastic constant. Therefore, the resultant liquid crystal composition is suitable for a high-speed liquid crystal device. However, the composition has a high cross-over frequency.
A liquid crystal used in a printer or the like has a period of 2 msec or less given to control a light-transmitting state (ON) and a light-shielding state (OFF) of one liquid crystal optical shutter in order to perform printing at a practical speed (e.g., about 10 A4 size sheets per minute). Such an optical shutter must be driven by using an RF electric field having a relatively high frequency, e.g., 100 kHz or more, and preferably 150 kHz or more.
The cross-over frequency changes depending on temperatures. In order to stably operate the liquid crystal optical shutter, the frequency of the RF electric field applied thereto must be sufficiently higher than the cross-over frequency.
The liquid crystal composition of the prior U.S. patent application requires a higher frequency of the RF electric field applied to the liquid crystal composition since the cross-over frequency is still high. When the frequency of the RF electric field is high, a large amount of RF current flows between opposite electrodes of the liquid crystal device through an equivalent capacitor. For this reason, much dielectric heat is generated. Further, Joule heat is generated by the resistances of the electrodes and the lead wires connected to these electrodes to apply the RF voltage thereto. When heated, the operating characteristics of the liquid crystal device are degraded. In addition, since a large amount of RF current flows, power consumption is undesirably increased. A complicated electronic circuit is required to generate a drive signal for applying the RF electric field. In this manner, a liquid crystal composition used in the liquid crystal device driven by the two-frequency addressing scheme must exhibit a typical dielectric dispersion phenomenon, respond at high speed at a relatively low voltage, and have a low crossover frequency.